The mechanism underlying alcohol addiction remains obscure, although it is well documented that the mesolimbic dopamine (DA) system, originating from the ventraltegmental area (VTA) and projecting to the nucleus accumbens (NAc), plays a critical role. The output of VTA DA neurons is normally constrained by powerful GABA-mediated synaptic inhibition. However, little is known about the factors that may govern this important GABAergic synapses. Our long-term goal is to elucidate the regulatory mechanisms controlling the GABAergic synapses as a prerequisite for the development of new therapy of alcoholics. The specific hypothesis behind the proposed research is that the glycine receptors (GlyRs) in the mesolimbic system play a major regulatory role that controls the GABAergic synapses on VTA DA neurons and ethanol intake. Preliminary electrophysiological evidence from our laboratory indicates that GlyRs exist on the GABAergic terminals, which make synapses on VTA DA neurons. Activation of these GlyRs reduces GABAergic transmission and increases VTA DA cell firing. In addition, recent in vivo studies indicate that microinjection of glycine into NAc, or system administration of ORG 25935, an inhibitor of glycine transporter 1, decreases ethanol intake. In this project, we will conduct experiments on rats which are trained for self-administration of ethanol with two-bottle protocol. Specific Aim #1 will assess the effects of VTA GlyRs on voluntary ethanol intake. We will determine the effects of intra-VTA injection of the agonist and/or antagonist of GlyRs on ethanol intake and on NAc DA levels. Specific Aim #2 will characterize the cellular mechanisms of glycine's action. We will assess the electrophysiological signals (e.g. GABAergic inhibitory synaptic currents and action potentials) recorded from VTA DA neurons in brain slices of rats with a long history of ethanol drinking. The result of this multiple disciplinary study may gain new insight into glycine's effects on the brain reward pathways and could lead the development of new therapies of alcoholics. PUBLIC HEALTH RELEVANCE: The mechanism of alcohol addiction remains obscure. Our preliminary studies found that glycine in the mesolimbic system plays an important role in regulating ethanol drinking. This project will investigate the cellular mechanism by which glycine regulates voluntary ethanol drinking. This proposal will bring important information to design rational pharmacotherapeutic interventions in alcoholism.